The present invention refers to a circulation mixer for bulk material, and in particular to a circulation mixer with at least one riser pipe accommodated in a container with funnel-shaped bottom part for pneumatically conveying the bulk material from the bottom part back to the upper section of the container.
The use of such circulation mixers for freely flowing bulk material is known. The riser pipe extends centrally within the container with its lower opening being e.g. at the same level as the outlet opening of the container. The diameter of the riser pipe is selected in such a way that an annular gap is defined between the container wall and the lower opening of the riser pipe so as to allow the bulk material to flow into the space below the opening of the riser pipe. Air is injected through the outlet for conveying the bulk material from the space below the riser pipe through the latter to the upper section of the container and thus provides the circulation of the bulk material.
A circulation mixer of this kind has the drawback that it is not suitable for bulk material which does not freely flow like e.g. powdery or dusty bulk material. When using such more or less cohesive bulk material, bridges are formed in the narrow cross sectional areas, in particular in the annular gap between the lower opening of the riser pipe and the container wall. In addition, lump-like accumulation of material is encountered in the space below the opening of the riser pipes resulting in malfunction of the circulation mixer because the aggregated material cannot be conveyed or can be conveyed only at great expenditure back through the riser pipe to the upper section of the container.
A proposal to enlarge the cross section of the annular gap in order to prevent a formation of bridges is not feasible since the annular gap serves simultaneously as seal for the circulating air so as to make sure that the air is solely guided through the riser pipe and thus the cross sectional area of the annular gap must be kept within narrow limits.